The present invention relates to an organic light emitting element and a display apparatus using such an element.
In an active matrix organic light emitting display apparatus, two to four drive elements including thin film transistor switching elements and capacitance are connected to an organic light emitting element which forms each pixel. Thus, all pixels can be turned on over one frame period. As a result, it is not necessary to raise the luminance and it becomes possible to prolong the life of the organic light emitting element. Therefore, it is considered that the active matrix organic light emitting display apparatus is advantageous to a larger screen with higher definition.
On the other hand, in an organic light emitting display apparatus in which emitted light is taken out from the back side of a substrate, an aperture ratio is limited provided that an active matrix having a drive unit between the substrate and organic light emitting elements is used. Especially in a large-sized display, there is a problem that it is necessary to widen the width of a power supply line in order to reduce the luminance variation between pixels caused by a voltage drop on the power supply line and the aperture ratio becomes extremely small.
An active matrix type organic light emitting display apparatus having an upper electrode made transparent and a structure for taking out emitted light from the upper electrode side, i.e., having a top emission structure is considered to be effective to solve the problem. In this apparatus, there is no drive unit on the upper electrode for taking out emitted light, and consequently the aperture ratio can be improved by leaps and bounds.
A technique of forming a thin film layer for hole injection having a large work function between an anode and an organic layer in order to improve light emission characteristics by increasing a hole current in an organic light emitting element having such a top emission structure is described in JP-A-2002-198182. A technique of forming an electron injection layer between a cathode and an organic layer is described in JP-A-2003-272867.
FIG. 9 shows a single pixel sectional view of an organic light emitting element having a top emission structure. In this organic light emitting element, a first electrode 115 serves as a cathode and a second electrode 125 serves as an anode. The element has a lamination structure including the first electrode 115, an electron injection layer 124, an electron transportation layer 123, a light emission layer 122, a hole transportation layer 121, a hole injection layer 129, a second electrode 125 and a protection layer 126.
The manufacturing process of the element having the top emission structure will now be studied. The first electrode 115 is patterned by wet etching or the like in a photolithography process. At this time, the surface is insulated or contaminated. If an electron injection layer is formed on the insulated or contaminated first electrode 115 in this case, the light emission efficiency falls or light is not emitted, resulting in a problem of lowered lighting reliability. Also in an element having a structure reversed in anode and cathode positions, this problem occurs in the same way.
According to JP-A-2002-198182 and JP-A-2003-272867, the electron injection layer or the thin film layer for hole injection is formed between the anode and the organic layer. However, it does not function to reproduce the efficiency of injection into the electron injection layer or the hole injection layer in the insulated or contaminated first electrode. The problem is not solved.